Funds are requested for the renewal of work supported by current grant. Visual receptive fields of single cells are to be determined by a method, termed Alopex, that uses positive feedback to generate visual patterns. The computer controlling this feedback uses and algorithm based on the cell's response, and is designed to make the pattern converge on a stimulus that maximizes the response. The proposed experiments will use the methods and extensive instrumentation developed under current grant. These will be applied, initially, to the fields of retinal ganglion cells of frogs by extra-cellular monitoring from afferent fibers in the tectum.